OK boys, I'm 47, here is some advice for you.
If a girl dumps you, do not ask why, do not make any attempt to keep her. Do not ask her friends about her, never call her.
This will go against your very nature and you may slip up but you must do this for 3 simple reasons.
You will not likely find "the one" at such a young age, you can try but they are bound to end up being temporary at least until you reach your mid to late twenties if not older.
You must keep your dignity. If you do not, you will absolutely lose the girl and your self respect both.
Old men are often still haunted, not by the one's that left, but what fools they were trying to hold onto them.
The more desperately you chase something, the more desperately it runs away from you. Whether you end up moving on or having a chance to reconcile, both hinge on the same exact action. You have to go on and be OK without them.
You do not have to be a jerk. The opposite of love is not hate, it is apathy.
So go on and live your life without them and remember a lot of old men also think about how nice it might be to be alone.